Which upgrade first : Cartridge or Phono preamp ?

I am trying to get back into vinyl. I just bought a Pro-Ject III ( Ortofon OM 5E )and a Pro-Ject Phono Box II and the combination sounds pretty good. What will make a bigger difference a different cartridge ( $ 200-250 new or used )or a Phono preamp ( $ 500-600 new or used ) ?
Many thanks
At this point, your phono preamp is better than your cartridge. Further upgrading your phono pre at this point would mostly point out the weaknesses of your cartridge. Move up to an Audio Technica AT150MLX or Ortofon 2M Blue cartridge, which should present a nice juicy signal for your Phono Box II to work with, but check the arm/cartridge database to make sure of compatibility.